This invention relates to apparatus for measuring electrostatic charge levels. More particularly, it involves an improvement in an electrostatic voltmeter, more commonly referred to as an electrometer.
Electrometers have been used in a wide variety of applications. They have found increasing use in the xerographic process wherein copies are formed from a latent electrostatic image on a charged photoreceptor. Satisfactory operation of the highly complex and sophisticated present day reproduction machine depends to a great degree on the ability to adjust the machine components for optimum performance. The electrometer, which has the capability of measuring electrostatic charge levels on the machine photoreceptor without touching its surface, is used to read the charge on the photoreceptor. On the basis of such reading, service personnel or automatic control circuitry can accurately adjust certain components affecting the charge level on the photoreceptor, e.g., the charging corotron, exposure duration, and developer bias, etc.
Currently available electrometers are either of the alternating current (A.C.) type or direct current (D.C.) type. In an A.C. type, an alternating signal is generated by the probe sensor element which views the test surface. This type of signal may be created, for example, by a rotating shutter or vane designed to periodically expose the test surface being measured to the entire probe element. The resulting A.C. signal is then processed to provide an appropriate readout. Unfortunately, known A.C. electrometers have a slow response time which can be extremely detrimental especially when automatic control of the machine components is employed, e.g. under microprocessor control.
In the D.C. electrometer, the probe sensor element is continuously exposed to the test surface. Hence, a D.C. signal is impressed on the sensor element which is amplified by a direct current amplifier. The output of the D.C. amplifier is then similarly processed to provide a readable output voltage, for example, by a voltmeter. The D.C. electrometers have considerably better speed of response than the A.C. electrometers. However, its output is subject to a phenomena known to those skilled in the art as "drift", which is due to the change in the balance of electronic components, etc. Since the output of the D.C. electrometers does not remain constant, an accurate reading of the electrostatic charge level on the test surface is difficult to measure over a significant period of time.